A small recommendation

Hello there,

recently i have gotten interest in the whole chip amp euphoria. I have built other amps before but never one of this sort. To test it out i built one based on this very simple design: Mick Feuerbacher Audio Projects
i understand it is probably not the best design option on stability due to its minimalisticity (this word probably doesnt exist but who cares), but i made it to check it out. The thing is... i really like this little bugger. So i thought of connecting it to my computer. I do have one small doubt though: I dont know what is the impedance for computer sound cards. Is there any modification i should make to complete my intended plan?

Another question: Should i worry about this design? Any urgent modifications in plan?

thanks in advance...
 
Read the LM3886 datasheet from start to finish - tells you a lot about what optional components do and why you may need them.

Some alternative designs:

This one covers pretty much every eventuality and then some - http://www.romanblack.com/gainamp/ampsch03.jpg

This one's interesting (no DC or capacitive load protection, no startup mute delay, bizarre schematic) - http://www.eleccircuit.com/wp-conte...6-gainclone-2x-68w-full-amplifier-project.gif

And this one's inexplicably popular (has DC protection but nothing in the negative feedback loop and no capacitive load protection) - http://electronics-diy.com/schematics/588/schematic-amp.gif

DC protection is what it says, if your source has significant DC bias, it can cause noise when powering on and off (popping speakers) and will cause distortion at high volume levels and make the chip generate heat excessively.

When there's some DC filtering on the negative feedback loop, the chip's own internal DC bias (which is usually very small) is prevented from being amplified.

You can also attach high frequency filtering on the input and negative feedback loop to reduce electromagnetic interference such as from fluro lights an switches and help keep the chip from self-oscillating under stressful or unusual conditions.

Capacitive load protection (Zobel network) prevents the amp going into spasms if you're driving very strange speakers or using long lengths of bizarre specialist cross woven speaker cable that's very capacitive.

A mute delay thing is where the pin 8 mute function is turned on for a second on powering the chip up (via a capacitor) - very useful if you have a preamp that takes a second to stabilise or are getting some start-up noise from your power supply, otherwise you can take it or leave it.


And the word is 'minimalism' :)

PS: no matter how you look at that copper block amp, he is driving it with a playstation. There may be a straightjacket just outside one of those photos...
 
Last edited:

albireo13

Member
2010-02-13 3:41 am
This amp has no DC rejection. Any DC on the input will be gained-up along with the audio signal. Something to consider. Therefore, there will likely be significant DC on the output. Additionally, this DC will vary with the volume POT.
Now, your source may have AC-coupling in its output but, that's not clear here.

Many designs incorporate a Cap in series with Rg. This causes the DC gain to be unity, as compared to the AC gain of (1+Rf/Rg). Alternatively, a DC-blocking cap at the input (in series with POT R1) will remove the input DC entirely. In any case, these caps bring with it charge-storage effects which need to be managed during power-up and power-down.

Rob
 
Yeah, i think the way to go is to solve the DC unwants of it... I finally read the datasheet (to be honest i was lazy to do so until it became inevitable) and agree with albireo13. The cap with the pot will solve it entirely with no danger of sound downgrades. In any case, i think this little guy can be quite useful for some minor applications.

Thanks for the help.